SUPERMAN: Neo-classic costume, much like Routh's supersuit, sans "lines".
BATMAN: An all-black Batsuit. Given that this movie would need to put him in perspective next to the other, more brightly colored heroes, I'd like to see the suit be as creaturelike and batlike as possible. Lots of sharp edges. Lenses in the cowl, etc.
WONDER WOMAN: Gold/silver armor and a leather cullotte. Armor and helm for later in the film.
J'ONN J'ONNZ: Black or dark blue or possibly dark green Martian "armor" that incorporates the classic "X" chest design, and a long, flowing black or dark blue cloak. No buccaneer boots, but his legs and feet would be covered. J'onn's armor and cloak, being Martian and organic, would morph along with him, into his civilian clothes or costume.
GREEN LANTERN: All black jumpsuit with raised metallic green lantern insignia. The aura would form most of the interesting visuals of Lantern's costume. In addition, if the GL is Kyle Rayner, the character could be constantly switching costumes or using variations on his, as he does in the comics.
THE FLASH: Padded, quasi-armored red supersuit and raised lightning bolt insignia with lensed helmet with earpieces.
AQUAMAN: Some combination of robes, cloak and armor. He could have battle-armor that resembled his classic look, incorporating golds and green colors.